Saliva ejector are used by the dentists to provide continued ejection of saliva out of the mouth of patients during dental treatment. The most common type of ejector is portrayed in FIG. 1. It comprises a flexible tube which can be bent to provide a hook over the lower lip of the patient. The hook is structurally maintained by a copper wire, which is built inside the wall of the flexible tube. Internally the flexible tube provides a channel for moving away waste liquids entering the tip. The flexible tube usually provides, at one end, a standard diameter for plugging directly into the tube which extends from the machine generating the vacuum and connecting with the evacuation system.
At the other end of the flexible tube fits the ejector tip as shown in FIG. 1, which generally provides openings through which saliva, blood, water, and other waste products from the patients' mouth pass into the flexible tube.
Various designs of saliva ejector tips are being commercially offered and are tried by the dentists in the hope of finding a satisfactory ejector. The criteria are mainly three: (1) not to suck up the patients floor of the mouth, tongue or other sensitive tissue inside the mouth; (2) not to clog up easily; and (3) to provide a standard diameter at the end so that it can plug into the machine's tube, without the need of a home-made adaptor.
One type of tip provides a coiled tubule having its end open and further having holes in between the convolutions so that the tongue and soft tissue of the mouth can be protected. The drawbacks of this design are: (1) the opening at the end is positioned away from the floor of the mouth where porcelain and metal solid particles accumulate; (2) the internal friction is excessive because of its small diameter and relatively long length, thereby providing slow service while being susceptible to clogging by small particles; and (3) it does not provide standard diameter at the end.
The conventional type, shown in FIG. 1, also provides drawbacks, particularly in the area of sucking up soft tissue and clogging.
From the engineering point of view the sucking force F that is applied by a hole at the tip to the soft tissue of the mouth, expressed in pounds, is equal to EQU F=(P.sub.i -Pa).times.A (1)
where: P.sub.i and P.sub.a represent the pressure inside the flexible tube and atmospheric pressure, respectively, expressed in pounds per square inch, and A is the area of the hole expressed in square inches.
To keep the force F small, the holes must be kept small; but in this case there is more of a change for clogging. It may be noted that the above force F can be maintained, despite the fact that other holes at the tip may be open and conducting waste. This is because the vacuum (P.sub.i -P.sub.a) can be easily maintained by a strong source, as long as the flow of waste liquids remains slow. The low rate of flow is due to the resistance contributed by the viscosity of the waste fluids as they flow through the walls of the holes and the flexible tube. Once a hole grabs the tissue, it will hold on to it as long as the force F remains enough to overcome the low skin tension force.
A desirable tip, therefore should provide sufficient hole area for the fluid to be processed quickly; after the fluid is sucked air follows at a faster rate, having a much lower viscosity. Air quickly increases the internal pressure P.sub.i, thereby reducing the value of F to less than skin tension so that the soft tissue, if held by other holes, can be released. It is apparent, therefore, that more hole area is needed at the tip with a large portion of such area being recessed away from the region of direct contact with the soft tissue.
Still another desirable feature would be for the tip to have a large opening at the lower end and near the region where hard particles, such as grinds of teeth, porcelain and metal usually fall. Provision, however, must be made to protect the patient's tissue f25 m a substantial force F, which such a large hole would provide.
A third desirable feature in a saliva ejector tip would be to provide control of the total area of openings versus the force F itself, so that a large F, indicating substantial sucking of soft tissue, could trigger a process of automatic increase in area not in contact with soft tissue.